This study proposes to continue the follow-up for risk of breast cancer of cohorts of 3,033 women given DES in pregnancy and a matched cohort (matching by age and source of prenatal care) of 3,033 women unexposed during pregnancy. At the last analysis as of August 1, 1981, some 5,000 of the women were known alive and free of breast cancer. That analysis revealed an overall adjusted relative risk of breast cancer of 1.5 (95% confidence interval of 1.1 to 2.0). Cumulative risk over time was similar up to 25 years after exposure at which time the incidence curves for exposed and unexposed commenced to diverge. Since the previous analysis, considerable women have passed their 30th anniversary and more of exposure. With the proposed follow-up in this study, the women-years observation for 30 years or more post-exposure will more than double that in the previous analysis and will permit: (i) a more precise estimate of breast cancer risk, (ii) determination of whether the observed divergence of cumulative breast cancer incidence continues with further duration of follow-up, (iii) assessment of whether the increased breast cancer incidence risk is reflected in mortality and (iv) allow for estimates of risks of other cancers. The study consists of a multi-institutional, collaborative effort with data collection at three local sites and with a central data coordinating center. The study team consists essentially of those who undertook the previous assembly, tracing and followup of the cohorts and who possess the necessary data base to permit further follow-up. As with the previous follow-up efforts, subjects will be asked to complete and return questionnaires. For reported incident events of cancer, permission will be sought to obtain medical records and microscopy slides for central review by a pathologist. For reported deaths, the death certificates will be sought.